The upgrade which mentions Michal affects only to the performance improvements included in V.Ray 3.20.03, but not to this problem. I apologize for the misunderstanding.
I'm making some tests with grass presets and V-Ray RT 3.20.03 (ActiveShade), but cannot reproduce the problem. Anyway the error is generated by V-Ray, not by Forest. Unfortunately we have not access to the RT render engine.
Please, would you submit this issue directly to ChaosGroup ? They will be more helpful to identify the problem. Thanks.
Regarding the billboards, i'm checking it and you are right: don't render with RT. It may work, but applying several changes:
1) Remove "Forest Material" and use VRayMtl direclty.
2) It seems GIFs are not supported in RT: replace the .gif opacity map by a different format (i.e. png).
3) Apply a "MeshSelect" to the Forest object, so it's recognized as a valid mesh object.
The implementation of Forest in V-Ray RT is completely different from V-Ray Adv. That is the reason of these limitations.
The dissapearing issue is caused by the way that Nitrous works: in the latest Max releases, viewports are drawn in background using a separated thread. Since Forest generates differentes geometries for viewport and render, ocasionally Max may redraw the render state instead the viewport.
Unfortunately we cannot fix that. Anyway, if you click Display->Update All after closing ActiveShade, that should rebuild all Forest objects correctly.
I hope that helps.